| offset {stats} | R Documentation | 
An offset is a term to be added to a linear predictor, such as in a generalised linear model, with known coefficient 1 rather than an estimated coefficient.
offset(object)
object | 
An offset to be included in a model frame | 
There can be more than one offset in a model formula, but - is
not supported for offset terms (and is equivalent to +).
The input value.
For examples see glm and
Insurance in package MASS.